Thermally efficient micromachined device

ABSTRACT

A micromachined device for efficient thermal processing at least one fluid stream includes at least one fluid conducting tube having at least a region with wall thickness of less than 50 μm. The device optionally includes one or more thermally conductive structures in thermal communication with first and second thermally insulating portions of the fluid conducting tube. The device also may include a thermally conductive region, and at least a portion of the fluid conducting tube is disposed within the region. A plurality of structures may be provided projecting from a wall of the fluid conducting tube into an inner volume of the tube. The structures enhance thermal conduction between a fluid within the tube and a wall of the tube. A method for fabricating, from a substrate, a micromachined device for processing a fluid stream allows the selective removal of portions of the substrate to provide desired structures integrated within the device. As an example, the micromachined device may be adapted to efficiently react fluid reactants to produce fuel for a fuel cell associated with the device, resulting in a system capable of conversion of chemical to electrical energy.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Work related to the present invention was supported in part under DARPAContract No. F30602-99-2-0544. The government may have certain rights inthe invention.

CROSS REFERENCE TO RELATED APPLICATIONS

Not applicable.

TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY OF THE INVENTION

The present invention is generally directed to a micromachined devicefor efficient thermal processing of fluids. More particularly, thepresent invention is directed to a micromachined device for carrying outfluid processing with improved thermal efficiency. As used herein, fluidprocessing includes, for example, subjecting a stream of a fluid tochemical reactions, heating, cooling, filtration, adsorption,desorption, and/or phase changes (e.g., evaporation and condensation).The present invention may be used in, for example, the processing ofchemicals for purposes of power generation.

DESCRIPTION OF THE INVENTION BACKGROUND

The field of micromachined fluidic devices encompasses all systems thatprocess components (e.g., gases, liquids, solid particles (e.g., beads),complex molecules (e.g., DNA), and mixtures thereof and contain smallfeatures (minimum feature sizes smaller than 500 μm). Such micromachineddevices have been shown to be useful in many fields, including chemicaland biological analyses (e.g., capillary electrophoretic separations),small-scale chemical synthesis, and measurement of reaction kinetics.The smaller dimensions inherent in micromachined devices enablesignificantly smaller fluid flow rates, reduced system size, and, inmany cases, improved performance.

A variety of fluid-processing micromachined devices are disclosed in theprior art. For example, U.S. Pat. No. 6,192,596 provides an activemicrochannel fluid processing unit and a method of making the unit.Arrays of parallel microchannels separated by thermally conductive finsprovide the mechanism for heat transfer to or from fluids moving throughthe microchannels.

U.S. Pat. No. 6,193,501 discloses a microcombustor of sub-millimeterdimensions. A preferred embodiment includes a wafer stack of at leastthree wafers, with the central wafer housing a combustion chamber. Atleast one inlet and one outlet are included for the insertion ofreactants and the exhaust of a flame.

U.S. Pat. No. 4,516,632 discloses a microchannel crossflow fluid heatexchanger and a method for making the same. The heat exchanger is formedfrom a stack of thin metal sheets bonded together.

One possible application of fluid-processing micromachined devices is asportable electric generators. This application is promising because theenergy density of chemical fuels exceeds that of presently availablebatteries by approximately two orders of magnitude. However, to takeadvantage of the high energy density of chemical fuels and compete withbatteries for portable power applications, suitable efficient designsfor micro-sized fuel processors/generators that convert chemical toelectrical energy must be created. Fuel processors/generators usuallyrequire regions of high temperature in order to sustain the desiredreaction. The power consumed in sustaining those temperatures reducesthe overall efficiency of the system. As device dimensions becomesmaller, it becomes increasingly difficult to maintain the thermalgradients and thermal insulation required for efficient fuel processing.Often with existing micromachined fuel processing devices, significantlymore power is required to maintain the temperature within the hightemperature regions of the device than is contained in the fuel,precluding the device's use for portable power generation.

Thermal management is crucial to producing efficient devices designed tooperate with separate features held at different temperatures. Inparticular, thermal isolation of the reaction zone in micromachinedpower generation systems is paramount. For micromachined non-fluidicdevices, thermal management is achieved by simple thermal insulationusing long, thin and/or non-conductive supports, often assisted bypackaging in vacuum. Examples of non-fluidic micromachined devicesproviding thermal management include, for example, bolometers, such asthose disclosed in U.S. Pat. Nos. 5,021,663 and 5,789,753. However, thepresent inventors have concluded that micromachined fluidic devices addthree unique difficulties: the need for enclosed fluidic structuresconnecting the thermal regions; the potential of added heat flow throughthermal convection; and, frequently, a desire to include regions wherethe walls of the fluid conducting tube are held isothermal. Thus, asuccessful thermal management scheme for a micromachined fluidic devicemust include, in addition to certain known inventions and techniquesused in non-fluidic thermal devices, a means to provide fluidcommunication with high temperature regions without causing excessiveheat flow either through the static structure or through the movingfluid. It is frequently desirable to include in this scheme means toensure thermal uniformity in specific regions of the device.

Accordingly, it would be advantageous to provide a micromachined devicecapable of efficiently conducting a chemical process involving at leastone fluid, wherein a high temperature reaction zone is thermallyisolated from its environment. It also would be advantageous to providea micromachined device for conducting a chemical reaction involvingfluidic reactants and wherein operation of the device consumessubstantially less energy than can be produced from the fluid reactants.Such a device could be used as part of a portable electric generator.

SUMMARY OF THE INVENTION

The present invention addresses the above-described needs by providing amicromachined device for thermal processing at least one fluid stream.The micromachined device includes at least one fluid conducting tube,and at least a region of the fluid conducting tube has a wall thicknessof less than 50 μm.

The present invention further addresses the above-described needs byproviding a micromachined device for processing at least one fluidstream, and wherein the device incorporates at least one fluidconducting tube and at least one thermally conductive structure. Thethermally conductive structure is in thermal communication with a firstthermally insulating portion of the fluid conducting tube and a secondthermally insulating portion of the fluid conducting tube.

The present invention also addresses the above-described needs byproviding a micromachined device for processing at least two fluidstreams including a first fluid conducting tube, a second fluidconducting tube, and at least one thermally conductive structure. Thethermally conductive structure is in thermal communication with athermally insulating portion of the first fluid conducting tube and athermally insulating portion of the second fluid conducting tube.

The present invention additionally offers a solution to theabove-described needs by providing a micromachined device for processingat least one fluid stream, wherein the device includes a thermallyconductive region and at least one fluid conducting tube having at leastone thermally insulating portion. At least a portion of the fluidconducting tube is disposed within the thermally conductive region.

The present invention is further directed to a method for processing afluid stream. The method includes providing a micromachined devicehaving at least one fluid conducting tube having a thermally insulatinginlet portion and a thermally insulating outlet portion, and at leastone thermally conductive structure in thermal communication with theinlet and outlet portions. A stream of at least one fluid is introducedinto the inlet portion of the fluid conducting tube, and the fluidstream is processed within the fluid conducting tube. The thermallyconductive structure conducts thermal energy between the inlet andoutlets portions of the tube.

The present invention also provides a portable power generator includinga micromachined device constructed according to the present invention.The micromachined device includes at least one fluid conducting tube andat least one thermally conductive structure. The thermally conductivestructure is in thermal communication with a first thermally insulatingportion of a fluid conducting tube and a second thermally insulatingportion of a fluid conducting tube. A fuel cell is in fluidcommunication with the fluid conducting tube. A fuel is produced in thefluid conducting tube and is conveyed to the fuel cell, where it is usedto generate power.

The present invention also discloses a method for fabricating a devicefor processing at least one fluid stream. The method includes patterninga substrate to form at least one tube mold having walls accessible to anenvironment and to form at least one release pit not accessible to theenvironment, depositing a thin film to coat the walls of the tube moldbut not the release pits, and removing selected regions of the substrateusing a chemical etchant to form at least one fluid conducting tube.

Embodiments of the micromachined device of the present invention addressthe difficulties described above encountered in adapting such devicesfor use in portable power generation. Micromachined devices constructedaccording to the present invention may be adapted for conductingchemical reactions with fluidic reactants wherein the device consumessubstantially less energy than can be produced from the fluid products.These and other advantages of the present invention will be apparent onconsideration of the following detailed description of embodiments ofthe invention. The reader also may comprehend additional details andadvantages of the present invention upon making and/or using themicromachined device of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is a schematic representation of an embodiment of amicromachined device adapted for processing a fluid and constructedaccording to the present invention. FIG. 1( b) is a side view of theembodiment of FIG. 1( a). FIG. 1( c) is a top view of the embodiment ofFIG. 1( a).

FIG. 2 is a scanning electron micrograph taken at approximately 20×magnification of an embodiment of a micromachined device adapted forprocessing a fluid and constructed according to the present invention.

FIG. 3 is a scanning electron micrograph taken at approximately 100×magnification of a region of an embodiment of a micromachined deviceadapted for processing a fluid and constructed according to the presentinvention, and particularly showing posts according to the invention.

FIG. 4( a) is a schematic representation of an embodiment of the presentinvention incorporating posts. FIG. 4( b) is a top view of theembodiment of FIG. 4( a) further including passive fluidic stop valves.

FIGS. 5( a) through 5(k) illustrate a fabrication process for forming amicromachined device constructed according to the present invention.

FIG. 6( a) depicts a top view of an embodiment of the present inventionemploying the use of vacuum packaging, reflective coating andlow-emissivity coating. FIG. 6( b) is a side view of the embodiment ofFIG. 13( a) further showing the use of packaging layers.

FIG. 7 depicts an embodiment of a generally U-shaped thin-walled tube ofthe present invention.

FIG. 8( a) and FIG. 8( b) illustrate the results of a finite elementsimulation of fluid and heat flow in the inlet and outlet portions of afluid conducting tube in an embodiment of a fluid-processingmicromachined device constructed according to the present invention.

FIG. 9 is a graph showing simulated ammonia cracker and systemefficiency as a function of system power for a micromachined deviceconstructed according to the present invention and incorporating aconventional proton exchange membrane (PEM) fuel cell system.

FIG. 10( a) depicts a process of ammonia cracking that may be carriedout in a micromachined device constructed according to the presentinvention. FIG. 10( b) depicts a process of methanol reformation thatmay be carried out in a micromachined device constructed according tothe present invention. FIG. 10( c) depicts a partial oxidation of butaneas may be carried out in a micromachined device constructed according tothe present invention. FIG. 10( d) depicts a process of thermoelectricgeneration that may be carried out in a micromachined device constructedaccording to the present invention.

FIG. 11 is a graph depicting the resistance in a micromachined deviceresistive heater as a function of the ambient pressure for three varyingapplied voltages.

FIG. 12( a) is a reflection optical micrograph taken at approximately20× magnification of iridium catalyst impregnated in fluid conductingtubes in an embodiment of a micromachined device constructed accordingto the present invention. FIG. 12( b) is a transmission opticalmicrograph taken at approximately 50× magnification of iridium catalystimpregnated in fluid conducting tubes in an embodiment of amicromachined device constructed according to the present invention.FIG. 12( c) is a graph of quadropole mass spectrometer (QMS) signalintensity over time for ammonia cracking over iridium catalyst at anammonia flow rate of 4 standard cubic centimeters per minute (sccm) inthe thermally conductive region of a micromachined device constructedaccording to the present invention.

FIG. 13( a) is a photograph of a micromachined device adapted forprocessing a fluid and constructed according to the present invention.FIG. 13( b) is an image of the embodiment of FIG. 13( a) wherein thethermally conductive region is heated to a temperature of about 1832° F.(1000° C.).

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention provides a solution to the problem of maintainingthe thermal gradients and thermal insulation required for efficientfluid processing in micromachined devices. It is believed that thepresent inventors' solution to thermal management of micromachinedfluidic systems is an enabling technology for man-portable fuelprocessing and power generation devices. The application of the presentinvention to the fuel processing and power generation field could leadto development of micromachined portable power supplies providing energydensities up to at least one order of magnitude greater than state ofthe art batteries.

It is believed that the application of the present invention to thefield of man-portable electric generators should provide substantialadvantages. An order of magnitude improvement in the power supply energydensity for man-portable electric generators would have a significantimpact on the multi-billion dollar battery industry. It would also offerthe potential to create an entirely new market for technologies andapplications currently made impractical by the low energy density ofbatteries. In addition to the power field, the present invention haspotential utility in a wide range of otherwise unrelated fields,including, for example, small-scale chemical synthesis, chemical sensingand analysis, micro-refrigeration (e.g., for electronics orsuperconductors), microcalorimetry, thermophotovoltaic power generation,stirling cycles, catalyst testing, and a wide range of chemical andbiological laboratory applications, including biological assays,chemical and biological sensing, and high throughput screening.

As used herein, the word “fluid” refers to one or more gases, liquids,solid particles (e.g., beads), complex molecules (e.g., DNA), andcombinations thereof.

As used herein, a “micromachined device” has the meaning generallyunderstood in the art, and refers to devices including a structurehaving a minimum dimension of less than 500 μm.

As used herein, the word “post” means a hollow or solid elongatedstructure having a cross-section of any geometry. By way of exampleonly, and without intending to limit the present invention, thecross-section of a post may have a perimeter that is any closed shape,such as circular, oval, square, or rectangular shapes.

As used herein, the term “substantially isothermal” means that thevariation in temperature over the substantially isothermal region isless than one fifth of the variation in temperature over the entiredevice. Temperature variations within the device can be measured in avariety of ways apparent to those skilled in the art. One such approachwould involve thermal infrared (IR) imaging of the device. Another suchapproach would involve placing temperature sensors at varying locationsof the device.

As used herein, the terms “thermally insulating” and “thermallyconducting” define a relative difference in thermal conductance within agiven embodiment of the invention. Specifically, a thermally insulatingstructure is at east twice, and preferably 10 times less conductancethan a thermally conductive structure, as defined by the power requiredto sustain a given temperature difference across the structure. Theconductance may be determined, for example, by direct measurement or,where suitable, by modeling. By way of example, and in not intending tolimit the present invention in any way, “thermally insulating” tubes maybe obtained by providing tubes with thin walls, preferably less than 50μm in thickness.

As used herein, the word “tube” means a hollow structure having an innerperimeter and an outer perimeter, and a cross-section of any geometry,and which allows passage of a fluid therethrough. By way of exampleonly, and without intending to limit the present invention, thecross-section of a tube may have a perimeter that is any closed shape,such as circular, oval, square, or rectangular shape.

One embodiment of the present invention provides a single, thin-walledtube, which may be utilized, for example, in high-temperature fluidprocessing. The inventors have discovered that tubes with thin walls,preferably less than 50 μm and more preferably less than 5 μm inthickness are useful for such an application because the thin wallsprovide reduced thermal conduction down the longitudinal axis of tube,even if the tubes span regions of substantially different temperature.To further reduce thermal conduction, the tubes also are preferably madeof a thermally insulating material.

Tubes having such thin walls can be fabricated by many techniques knownin the field. A preferred technique described in detail below involvesfabricating a relatively solid mold with encased tubes, followed bythin-film deposition of the tube material, and then release of the tubesby etching the surrounding mold. This technique provides freestandingtubes with a wall thickness defined by the deposition thickness.

In one embodiment of a micromachined device of the present invention, atleast one thermally conductive structure is in thermal communicationwith a first thermally insulating portion of a fluid conducting tube anda second thermally insulating portion of the same fluid conducting tube.The two portions with which the thermally conducting structure is incommunication are at different temperatures, with thermal energy beingtransferred from the hotter portion to the cooler portion.

Conservation of mass requires that a device in steady-state operationhave exactly as much mass flowing into a region as out of the region.Consequently, the heat loss due to gas leaving a region can be minimizedby transferring that heat to the incoming fluid stream. An alternateembodiment of the present invention accomplishes that heat exchange byplacing thermally conductive structures between a thermally insulatinginlet portion of a fluid conducting tube and a thermally insulatingoutlet portion of a fluid conducting tube. Preferably, the tube isarranged in a U-shape with longitudinal axes of the inlet and outletportion oriented substantially parallel. Heat flow from the tube due toconvection is ameliorated by the conduction of heat from the outletportion, through which the high temperature reaction products flow, tothe inlet portion of the fluid conducting tube. Ideally, thermallyconductive structures are substantially isothermal during operation ofthe device and have minimum dimensions typically larger than those ofthe tube wall, and are constructed of a relatively highly conductivematerial in order to facilitate heat flow. Preferably, the thermallyconductive structures are comprised of silicon.

Another embodiment of the present invention provides a fluid conductingtube with at least one thermally conductive structure that is in thermalcommunication with a thermally insulating inlet portion and a thermallyinsulating outlet portion of the fluid conducting tube. These thermallyconductive structures further reduce convective losses, and thus allowfor fluid processing to be carried out at higher temperatures and/orlower flow rates. As an example, the thermally conductive structures maybe oriented substantially orthogonal to a longitudinal axis of a fluidconducting tube so as not to increase thermal conduction down the lengthof the tube, while allowing thermal conduction between the inlet andoutlet portions. It will be understood that the direction of fluid flowis substantially in the direction parallel to the longitudinal axis ofthe particular region of the fluid conducting tube. Multiple thermallyconductive structures spaced apart along the inlet and outlet portionsof the fluid conducting tube may be used to enhance heat exchange overany desired length of the tube.

In one embodiment of the present invention, posts may be included withinthe fluid conducting tube. The benefits provided by inclusion of theposts are detailed herein below.

An alternate embodiment of the present invention provides asubstantially isothermal thermally conductive region in which at least aportion of the fluid conducting tube is disposed. At least a region ofthe fluid conducting tube is thermally insulating. By way of exampleonly, and without intending to limit the present invention, the portionof the fluid conducting tube may be wholly or partially embedded orencased in the thermally conductive region. The thermal uniformity ofthe thermally conductive region helps to ensure uniformity of a reactiontaking place in the embedded or encased portion of the thin-walled tube,and thus improves selectivity of reaction products and/or improvesconversion. The thermally uniform region also may minimize materialconstraints by eliminating hot spots, maximize emission of radiation forthermophotovoltaic (TPV) applications, and/or maximize contact areaavailable for thermoelectric (TE) generation. Thermal uniformity is alsohighly desirable in analytical applications where it is critical thatcertain physical properties are measured at a known and uniformtemperature. It is to be understood that while the walls of the fluidconducting tube within the thermally conductive region are substantiallyisothermal during fluid processing, the fluids contained within the tubeare not necessarily substantially isothermal.

The present invention also recognizes that the fluid within the fluidconducting tube may be a poor thermal conductor, causing thermalgradients from the wall of the tube to the region of the fluid at thecenter of the tube. This non-uniformity can be reduced by includingstructures within the fluid conducting tube designed to decrease theaverage distance heat must be conducted through the fluid to reach thetube wall. By way of example, and not intending to limit the presentinvention in any way, an array of posts may be positioned within thetube, projecting from the tube's inner wall and into the tube's interiorvolume. The posts preferably are thermally conductive and preferablygreater in diameter than the fluid conducting tube wall thickness andpreferably are made from a relatively highly conductive materialrelative to tube wall material. The posts also may be used inconjunction with the above-described thermally conductive structuressituated between tube inlet and tube outlet portions to further enhanceheat exchange and/or recuperation between the outlet portion of thefluid conducting tube, which contains a fluid at high temperature, andthe inlet portion of the tube, containing a fluid at relatively lowtemperature. The posts may also be used in conjunction withsubstantially isothermal structures to improve the thermal uniformity ofthe fluid contained within the substantially isothermal structure.

In another embodiment of a micromachined device of the presentinvention, at least one thermally conductive structure is in thermalcommunication with a thermally insulating portion of a first fluidconducting tube and a thermally insulating portion of a second fluidconducting tube. The two portions with which the thermally conductingstructure is in communication are at different temperatures, withthermal energy being transferred from the hotter portion to the coolerportion.

Without intending to limit the scope of the present invention, oneembodiment combines multiple tubes with a common substantiallyisothermal thermally conductive region and/or thermally conductivestructures. This combination provides an additional improvement thatcould, for example, allow the heat generated by one reaction to be usedby some other endothermic reaction or reactions. For example, hydrogenmay be combusted in the presence of air to produce water. This reactiongenerates thermal energy, which may then be used, for example, to aid inthe decomposition of ammonia into hydrogen and nitrogen.

In embodiments of the present invention, the open ends of the tubes maybe encased in a common substrate for ease of connecting the invention tothe environment.

In embodiments of a micromachined device of the present invention, theencasing of the tube ends may be extended to produce a sealed chamberaround portions of the micromachined device. A sealed chamber would beuseful in controlling the ambient environment around the structure. Forexample, the chamber may be evacuated to reduce the conductive andconvective losses from the external surfaces of the micromachineddevice. Alternatively or additionally, the chamber may be filled with anaerogel to reduce the conductive, convective and/or radiative losses.Additionally, coating the interior surfaces of the chamber with areflective material and/or coating the exterior surfaces of themicromachined device with a non-emitting material would reduce theradiative losses.

Embodiments of a micromachined device of the present invention may beconstructed such that high temperature portions of the device are usedfor direct electrical energy conversion. This may be accomplished, forexample, by using thermoelectric (TE) or thermophotovoltaic (TPV)methods.

It is contemplated that in certain embodiments of the present inventionthe fluid conducting tube may have more than one inlet portion and/ormore than one outlet portion. Such tubes may be fabricated as generallydescribed herein with modifications appropriate to form the multipleinlet and/or outlet portions.

In certain embodiments of the present invention the fluid conductingtube can branch into more than one tube. In addition, one tube can beformed from a junction of at least two tubes.

By way of example, and without intending to limit the present invention,any combination of two or more of the following is contemplated:thin-walled tubes, thermally insulating tubes, posts, thermallyconductive structures, substantially isothermal regions, thermallyconductive posts, multiple tubes, multiple inlets and outlets, branchingof one tube into more than one tube, joining multiple tubes into onetube, encasement of the tube ends, packaging of the embodiment in asealed chamber with various characteristics, and/or inclusion of directenergy conversion devices. The present inventors also contemplate thecombination of these elements with other elements, devices, and featuresknown to be useful in fluidic devices. These include thermopiles useful,for example, in power generation, refrigeration, or temperature sensing.A selective or blackbody emitter, used to generate light from heat, maybe integrated into devices of the present invention for use in athermophotovoltaic generator. Various sensors may be integrated into themicromachined device, including resistive temperature sensors,photo-acoustic spectroscopy devices, IR spectroscopy devices,capacitance sensors, thermal conductance sensors, flow sensors, chemicalsensors, and any of various other sensors. Actuators may also beintegrated into the micromachined device such as, for example, resistiveheaters, electrodes for electrophoretic or electroosmotic flow,electrodes for electrochemistry, valves, pumps, and any of various otheractuators.

One embodiment of a micromachined device constructed according to thepresent invention includes at least one fluid conducting tube having athermally insulating inlet portion which carries fluid to anintermediate portion of the fluid conducting tube located in a thermallyconductive region. The fluid conducting tube also has a thermallyinsulating outlet portion which carries fluid away from the intermediateportion of the fluid conducting tube. Thermally conductive structurescontact both the inlet portion and outlet portion. Thus, the directionof fluid flow and the direction of conductive heat flow are separatelycontrolled. That is to say, the direction of fluid flow is in thedirection of the tube and conductive heat flow is at least partiallythrough the thermally conductive structures. This substantial separationof fluid flow and conductive heat flow is enabled by the largedifference in thermal conductance of the thermally insulating tubeportions and the thermally conductive structures, as defined previously.

This difference in thermal conductance is achieved by using very thinwall dimensions (below 50 μm) for the tube and larger dimensions for thethermally conductive structures. Preferably, additional thermalconductance difference is achieved by the use of different materials forthe thermally insulating tube portions and thermally conductivestructures, with the material used for the thermally conductivestructure having a higher thermal conductivity than the material usedfor the thermally insulating tube portions. The device can be packagedsuch that the inlet, outlet and intermediate portions are in a vacuumenvironment, using microfabrication techniques known to those ofordinary skill in the art. The device may also be packaged with IRreflectors lining the vacuum cavity and low emissivity coating on thedevice itself to minimize radiative heat losses. By way of example only,and without intending to limit the present invention in any way, asilicon substrate may be coated with an IR-reflective material such asaluminum.

In an embodiment of a micromachined device for fluid processingconstructed according to the present invention, cold fluid reactants areintroduced through a substrate at room temperature to a thermallyinsulating inlet portion of a fluid conducting tube. The fluid is thentransported to an intermediate portion of the fluid conducting tubewhich is encased in a thermally conductive region. A reaction occurs inthe intermediate portion of the fluid conducting tube. Importantly,however, the fluid may react while in the intermediate portion and/orany other portion of the fluid conducting tube. Following the reaction,the fluid products are transported to a thermally insulating outletportion of the fluid conducting tube. By providing one or more of thethermally conductive structures between and in contact with the inletand outlet portions, heat is transferred from the fluid products in theoutlet portion of the fluid conducting tube to the fluid reactants inthe inlet portion of the fluid conducting tube. Only a minimal amount ofheat is lost by way of thermal convection through the fluid conductingtube to the substrate.

One type of structure providing good thermal isolation from theenvironment is a suspended fluid conducting tube made of a thin filmceramic insulator (silicon nitride) attached to a supporting siliconframe. The fluid conducting tube is integrated with slabs of silicon forpurposes of thermal management. The silicon slabs are shaped and locatedto form thermally conductive and/or substantially isothermal structuresto enable maintenance of a local thermally uniform region and heatexchange between various process streams, allowing heat recovery betweenthe inlet and outlet portions of the fluid conducting tubes. The fluidconducting tubes may be further insulated using aerogels or can bevacuum packaged. The fluid conducting tubes are usually packaged withina sealed cavity in the substrate. The cavity can be filled with anaerogel to reduce thermal conductivity and minimize emission losses orit can be evacuated to minimize convective/conductive losses. The IRreflectors and/or low emissivity coatings described above may beutilized either alone or in conjunction with the vacuum and/or aerogels.

In one embodiment of the present invention, two U-shaped silicon nitridetubes may be placed in proximity to one another to form four channels.The general U-shape of the tube plays an important role in that itserves to minimize stresses in the tube which may occur as a result ofthermal expansion. By minimizing stresses, such untoward results as tubebuckling are avoided. Accordingly, while only a U-shaped embodiment isdiscussed herein, the present invention is not limited to a tube havinga U-shape, but rather it is contemplated that any stress-relieving shapemay be used. Under suitable conditions it is possible that other tubeshapes also may be used. The U-end (intermediate portion) of each tubeis completely encased in a single relatively thick layer of silicon (atleast 30 μm). This silicon enables heat exchange between theintermediate portions of the two tubes. Such heat exchange is desirablefor carrying out endothermic reactions using a chemical heat source froman exothermic reaction, rather than other forms of heating. One examplewould be for hydrogen combustion to be carried out primarily in theintermediate portion of one tube to provide the heat required formaintaining temperature of the reaction zones and heat for endothermichydrogen generation reactions carried out primarily in the intermediateportion of the other tube, such that net hydrogen is produced. Thethermal conductance properties of the silicon structure should provide asubstantially common temperature throughout the silicon layer. As such,the region of silicon or other thermally conductive material withinwhich the intermediate portion is encased may be referred to as a“substantially isothermal thermally conductive region.” Withoutintending any difference in meaning, the region usually is referred toherein as a “thermally conductive region.” By way of example only, andwithout intending to limit the present invention in any way, theaforementioned “other thermally conductive material” may include anymetal, such as copper. The inventors believe that providing such aregion would provide a substantial advantage in heat integration ofmultiple process streams in micromachined devices.

Between the thermally conductive region and the substrate, one or morethermally conductive silicon structures may be provided to span thefluid conducting tubes. The thermally conductive structures provide athermal link for heat recuperation between the hot fluids leaving thethermally conductive region through an outlet portion of a siliconnitride tube and the cold fluids entering the thermally conductiveregion through an inlet portion of the silicon nitride tube. A platinumheater/temperature sensing resistor (TSR) with four leads suspendedabove the tubes meanders over the thermally conductive region. As willbe understood by those of ordinary skill in the art, the heater/TSR isessentially a resistive heater. By measuring the voltage drop andcurrent across this resistor and knowing resistance at a referencetemperature, the temperature of the heater can be determined.Accordingly, the heater and the sensor may be a single device havingmultiple functionality or they can be separate devices. Multiple heatersand temperature sensors may also be included at various locations on thedevice. The materials used for construction of the heater/TSR are knownto those of ordinary skill in the art. Alternatively, heating may alsobe done radiatively or using thermoelectrics. There is no direct siliconpathway between the thermally conductive region and the surroundingsilicon chip (substrate).

The present invention also provides a portable power generator includinga micromachined device for processing a fluid stream. The generator maybe comprised of a micromachined device, such as is described in Examples3-5 below, wherein the device is in communication with a fuel cell. Apreferred product produced by the micromachined device is hydrogen,which may be used as a fuel in many known fuel cells. Alternatively,hydrogen and carbon monoxide may be produced by the micromachined deviceand used in certain fuel cells.

Another embodiment of the present invention is a refrigeration device,such that the thermally isolated structures are at temperatures lowerthan the environment. By way of example only, a Peltier cooling deviceconnects the thermally isolated tube region to the substrate. Applyingvoltage to the Peltier device causes heat to flow from the thermallyisolated tube region to the substrate, effectively lowering thetemperature of the thermally isolated tube region relative to thesubstrate and the environment. In one embodiment, thermally conductingstructures connect the thermally insulating inlet and outlet portions ofthe fluid conducting tube. The thermally conducting structures serve tocool the incoming fluid stream and heat up the exiting stream, improvingthermal efficiency. The thermally isolating tubes and the heat exchangedbetween the inlet and outlet stream serve to minimize the powernecessary to apply to the Peltier device and maintain the lowtemperature. Thus all discussions included herein which refer tohigh-temperatures should also be construed to envision operation attemperatures below the temperature of the environment.

One embodiment a micromachined device constructed according to thepresent invention may include fluid conducting tubes with at least onestatic fluidic mixing structure protruding inwardly from the tube wallsinto the interior of the tube. These mixing structures can be designedby those skilled in the art to improve mixing in the fluid and may beimportant for reactions as well as heat transfer. These mixingstructures may be positioned in an irregular pattern so as to result ina increased mixing behavior of the fluid within the tube. The mixingimproves thermal uniformity of the fluid within the tube and decreasesthe diffusion distance of fluid components to the wall. The staticmixing structures could also be used to mix at least two different fluidstreams.

The present invention also anticipates the possible inclusion ofheterogeneous catalyst into various portions of the micromachineddevice. The inner volume of the intermediate portion of the fluidconducting tube, which may be encased in the thermally conductiveregion, often must include catalyst. The posts or other equivalentstructures described above may also serve the function of catalystsupport in order to increase the catalyst area and decrease the distancefluid components must diffuse to contact catalyst. The micromachineddevice of the present invention also may include other known structuresfor catalysis in fluidic systems. Such known structures include, forexample, porous catalyst supports, closely spaced posts for use intrapping powdered catalyst or passive fluidic stop valves designed tolocalize catalyst deposited from solution. The space between a set ofsuch “closely spaced posts” is smaller than particle size of catalyst tobe retained.

Noble metal catalysts have been introduced successfully into the fluidconducting tubes by the inventors through standard wet impregnationtechniques. In this procedure a compound of the noble metal (e.g.,H₂PtCl₆ for platinum) is dissolved in water and wicked or pipetted intothe fluid conducting tubes. Passive stop valves, further describedbelow, can be used to determine the tube region wetted with catalystprecursor solution, and thus control the region of catalyst deposition.During evaporation of the water, the metal compound precipitates on thewalls of at least a region of the fluid conducting tube. This solid isthen reduced to a pure metal by reacting it with hydrogen at elevatedtemperature, as is known by those of ordinary skill in the art.

A number of techniques for packaging embodiments have been pursued bythe inventors. The use of glass sealing techniques has been explored.The greatest success has been found with a glass transfer tape used as apreferred adhesion layer between the substrate and packaging layers. Theglass tape has some benefits because it is thermally stable at hightemperatures, which helps in manufacturing flexibility.

One representative class of applications of the present invention is asa gas-phase chemical reactor designed to conduct high-temperaturereactions in an efficient manner in a small system. One particularapplication of a gas-phase reactor of this type is in fuel processingfor portable power generation, as described previously.

An embodiment of the present invention, designed for the specificapplication of portable fuel processing and power generation, is thegas-phase chemical reactor designated generally as 1, shownschematically in FIGS. 1( a)-(c). This particular embodiment consists ofa thermally conductive region 2 in which the chemical reaction mayoccur. The thermal conductivity of the material from which region 2 isconstructed and the geometry of its structure allow the region 2 to besubstantially isothermal when the reaction is occurring. The region 2 isconnected to a substrate 3 by two fluid conducting tubes 5 and 6 inorder to allow two separate fluid streams to be passed through theregion 2. Fluid flows substantially along the longitudinal axis of eachfluid conducting tube 5 and 6. For ease of presentation, going forward,the discussion will focus on fluid conducting tube 5. However, thefollowing discussion applies equally to tube 6. Tube 5 is generallyU-shaped and includes a thermally insulating inlet portion 5 b, athermally insulating outlet portion 5 a, and intermediate portion 5 c.As shown in FIG. 1( a), the tube 5 has a rectangular perimeter. It willbe understood, however, that tube 5 may have a cross section that iscircular or of any other closed shape. The inlet portion 5 b and outletportion 5 a of the fluid conducting tube 5 are thermally connected byseveral thermally conductive structures 7, as described above, for heatrecovery. The inlet portion 5 b of the fluid conducting tube 5 isthermally connected to the outlet portion 5 a of the fluid conductingtube 5 by way of thermally conductive structures 7. The outlet and inletportions 5 a and 5 b of the fluid conducting tube 5 are in communicationwith the substrate 3 and may be accessed through ports 4 in thesubstrate 3. Fluids may be introduced to the inlet portion 5 b orremoved from the outlet portion 5 a.

The fluid conducting tube 5 is fabricated from chemical-vapor-deposited(CVD) silicon nitride, preferably using the mold and release techniquedescribed below, and preferably has a wall thickness of 0.1-3 μm. Thevery small wall thickness inhibits conduction of thermal energy alongthe tube 5. The thermally conductive structures 7 and thermallyconductive region 2 are composed of silicon, which has high thermalconductivity. The thickness of the minimum dimension of the thermallyconductive structures 7 and the thermally conductive region 2 variesfrom 20-200 μm. The region of the inlet portion 5 b and the region ofthe outlet portion 5 a of the fluid conducting tube 5, which extend fromthe substrate 3 to the thermally conductive region 2, are eachapproximately 3 mm long and the inlet and outlet portions are spanned by3-14 thermally conductive structures. It to be understood that whileadditional structures improve heat recovery, decreasing marginal utilityis seen with each additional structure. The thermally conductivestructures 7 are 50-500 μm wide (measured parallel to the longitudinalaxis of the fluid conducting tube 5) and fully surround a region of theinlet portion 5 b of the fluid conducting tube 5 and a region of theoutlet portion 5 a of the fluid conducting tube 5. The thermallyconductive region 2 is approximately 2 mm long and 1.6 mm wide. Withinthe region 2, the inlet portion 5 b of the fluid conducting tube 5 andthe outlet portion 5 a of the fluid conducting tube 5 are connected bythe intermediate portion 5 c of the fluid conducting tube 5, which maybe a simple “U” shape that is encased within the region 2 such that heatgenerated within the intermediate portion 5 c will be conducted into andthroughout the region 2. The fluid conducting tubes 5 and 6, thermallyconductive structures 7, and thermally conductive region 2 preferablyare packaged in vacuum in order to minimize conductive and convectiveheat losses to the surroundings. Region 2 is also intended to conductheat between the intermediate portions of tubes 5 and 6. Thermallyconductive structures 7 are used for heat recuperation between the inletportion 5 b and outlet portion 5 a of tube 5.

FIG. 2 is a scanning electron micrograph at 20× magnification of anembodiment of the invention constructed as generally shown in FIGS. 1(a)-(c). For that reason the element numbering used in FIGS. 1( a)-(c) isused in FIG. 2. A resistive heater trace 9 which also serves as atemperature sensor, may be integrated into the structure to allow forelectrical heating (for example, during startup), as well measurement oftemperature during operation. The resistive heater trace 9 is disposedon an exterior surface of the thermally conductive region 2 and heatsthe region 2 to the desired operating temperature. The resistive heatertrace 9 has a plurality of electrical leads 10. The inlet portion 5 band 6 b of the fluid conducting tube 5 and 6 and the outlet portion 5 aand 6 a of the fluid conducting tube 5 and 6 are disposed below theleads 10. The intermediate portion (not shown) of the fluid conductingtube 5 and 6 is encased in the thermally conductive region 2. Thermallyconductive structures 7 span the gap between the inlet portion 5 b andoutlet portion 5 a of tube 5 and the gap between the inlet portion 6 band outlet portion 6 a of tube 6.

FIG. 3 is a scanning electron micrograph of a fluid conducting tube 5showing posts 15 projecting from the tube walls and disposed within theinner volume of the fluid conducting tube 5. The posts 15 are disposedbetween the bottom wall 5 f of the fluid conducting tube and the topwall (not shown) of the fluid conducting tube. A side wall 5 e of thefluid conducting tube is also shown, removed by a gap from the posts 15.These posts 15 may be disposed in one or more of the inlet portions ofthe fluid conducting tube, the outlet portion of the fluid conductingtube, and the intermediate portion of the fluid conducting tube. Thearrangement of the posts 15 in FIG. 3 is meant by way of example onlyand in no way indicates any limitation on the configuration of postscontemplated by the present invention.

Additionally, non-limiting examples of post 15 disposition are seen inFIG. 4( a) and FIG. 4( b). FIG. 4( a) is a schematic representation ofan embodiment of the invention and particularly shows a possibleposition of posts 15 disposed along with thermally conductive structures7 in the inlet portion 5 b of the fluid conducting tube 5. Posts 15 willenhance conduction of thermal energy from the fluid in the vicinity ofthe thermally conductive structures 7 to the wall of the tube 5, whereheat may then be transferred to regions of lower temperature through thestructures 7. Thus, the combination of posts 15 and structures 7 willfurther assist in preventing dissipation of heat away from theintermediate portion 5 c of tube 5. Posts 15 are also disposed in theintermediate portion 5 c of the fluid conducting tube 5. Theintermediate portion 5 c is encased in the thermally conductive zone 2.

FIG. 4( b) is a schematic top view of the embodiment of FIG. 4( a). FIG.4( b) shows two fluid conducting tubes 5 and 6. The embodiment shown inFIG. 4( b) also includes passive fluidic valves 11 for catalystdeposition within the tubes. The stop valves 11 serve to control fluidflow within the fluid conducting tube 5 such that catalyst is depositedupon the walls of the fluid conducting tube 5 upon evaporation of acatalyst containing fluid.

The device of the present invention may be fabricated according to themethod illustrated in FIGS. 5( a) through 5(k). The goal of the firstseveral steps of the method is to provide a structure that will serve asa mold onto which the material of which the tubes are made is deposited.The mold will not only include structures that will define the tubes,but also will include buried release pits, which will not be coated withmaterial by the tube deposition process and will later serve to definethe thermally conductive structures and the thermally conductive region.Release pits are critical to the fabrication process. Release pits arecavities within the mold which are inaccessible during deposition oftube material, but are accessed during removal of parts of the moldusing a chemical etch. Essentially they define thickness of the tubemold side walls, which allows some mold sidewalls to be completelyremoved while leaving mold side wall material in other regions, asdescribed in the following paragraphs.

The present discussion will focus on a process utilizing at least twosubstrates. However, a number of approaches may be used to provide themold structure, including the use of only one substrate. More than twopatterned or unpatterned substrate layers of varying thicknesses couldbe laminated together to achieve this structure. Herein described is atwo-substrate process having several variations. The inventors havesuccessfully used the two-substrate process to form the tube moldstructures used to create fluid conducting tubes of micromachineddevices of the present invention.

The two-substrate process begins with a thin substrate. The materialsused for the substrate may include, for example, SOI (silicon oninsulator) wafers and plain silicon wafers. An SOI wafer allows a moreuniform definition of tube shapes in the subsequent processing steps,but a plain silicon wafer also works well. The substrate 103 ispatterned with relatively shallow holes on one side, as shown in FIGS.5( a)-5(c). A dry or wet etchant may be used to pattern the substrate103. In the present example, a deep reactive ion etch (DRIE) process isused to pattern the silicon.

The substrate 103 is then patterned from the opposite side such that thesubstrate 103 is patterned entirely through in regions of the initialshallow pattern. This patterning can also be accomplished using anysuitable wet or dry etchant, and the DRIE process was again utilized inthis example. This patterning step is crucial for establishing the finaldevice geometry. The shape of a tube mold 135 a and 135 b and a secondtube mold 140 a and 140 b is determined in this step, as seen in FIGS.5( d) and 5(e). FIG. 5( e) is a cross section of the device of FIG. 5(d) at positions I-I, II-II, III-III and IV-IV. For ease of presentation,going forward only tube mold 135 a and 135 b will be discussed, unlessindicated otherwise. Any discussion of tube mold 135 a and 135 b,however, is equally applicable to tube mold 140 a and 140 b. The releasepit 120 geometry is also determined at this step. The shape of the tubemold 135 a and 135 b and the shape of the surrounding release pits 120determine the thickness of the mold walls. Varying this thickness,combined with a subsequent etch to remove sections of the mold walls122, determines the final device shape. For example, as seen in FIG. 5(e), the walls at position II-II of the tube mold, are much thinner thanthe walls of the tube mold at position II-II. Accordingly, if both areasare exposed to the same etchant for the same time period there will bewall fragments remaining at position II-II, but not at position II-II.

It is also during this second pattering step that features extendingfrom the tube walls into the tubes can be provided. FIG. 5( d) showspassive fluidic stop valves 111, which aid in catalyst deposition laterin the process. The fluidic stop valves are formed using the DRIEprocess as set forth above. Similarly, posts made out of substratematerial can be patterned throughout the tube mold space using the DRIEprocess. The posts increase surface area of the catalyst and assist inachieving uniform heat distribution from the fluid in the center of thetube out to the tube wall. Fluidic mixing structures made out ofsubstrate material may also be patterned throughout the tube mold spaceusing the DRIE process. These mixing structures can improve mixing ofthe fluid and may be important for reactions as well as heat transfer.The ability of this process to utilize almost arbitrarily complex devicegeometries without impacting the overall processing is important fordesign optimization.

Following the patterning of tube molds and release pits, the tube moldand the release pits are capped with a substrate material roof 125,i.e., the fourth wall, as shown in FIGS. 5( f) through 5(i). FIG. 5( g)is a cross section of the device of FIG. 5( f) at positions I-I, III-IIIand IV-IV, while FIG. 5( i) is a cross section of the device shown inFIG. 5( h) at positions I-I, II-II, III-III and IV-IV. Again, the wallsat position II-II of the tube mold, are much thinner than the walls ofthe tube mold at position III-III. Accordingly, if both areas areexposed to the same etchant for the same amount of time, there will bewall fragments remaining at position III-III, but not at position II-II.Several strategies can be used for the capping layer. A thick substratematerial roof 125 can be bonded to the original substrate 103, and thenthinned back to desirable thickness through either chemical ormechanical machining. Alternatively, an SOI wafer can be bonded with thethin silicon side down. The buried oxide in this case provides an etchstop for the chemical thin back of the substrate material roof 125,leaving a thin roof layer defined by the original thickness of the SOIlayer. In another alternative, a substrate of desired thickness can bebonded directly to the patterned substrate 103.

Silicon capping substrates only 20 μm thick have been utilized in thefabrication process, as well as SOI wafers. Importantly, the cappingsubstrate thickness determines the thickness of the top walls instructures 102 and 107, as seen in FIG. 5( i). FIGS. 5( f) and 5(g) showthe process with an SOI wafer used as the substrate material roof 125before thin back, while FIGS. 5( h) and 5(i) show the process with anSOI roof 125 wafer after thin back.

The capping step completes fabrication of the tube mold substrate withembedded release pits. As mentioned above, this fabrication can also beachieved in a variety of other ways. For example, more than twosubstrate layers can be laminated together to build up the moldstructure and release pits. The structure could also be built usingelectroplating with sacrificial layers or other rapid prototypingapproaches.

Following the capping step, the tube material 130 is deposited on allthe exposed surfaces of the mold, as also illustrated in FIGS. 5( f) and5(g). The inventors have utilized a low pressure CVD technique todeposit silicon-rich silicon nitride films 2 μm thick. Electroplating orelectroless plating, wet deposition of ceramics using sol gels, andother processes known to those skilled in the art may also be used toachieve the deposition of other tube materials. A key feature of thisstep is that the release pits 120 have been sealed in during the cappingstep, and are not exposed during the deposition step. The tube materialis therefore not deposited inside the release pits 120. Once the releasepits 120 are exposed in subsequent processing, the substrate 103surrounding these release pits 120 can be selectively removed whileleaving the tube formed in the tube mold 135 a and 135 b intact. Thisremoval is accomplished by using a liquid or gaseous etchant thatselectively removes the substrate but not the tube material.

The next step, shown in FIGS. 5( h) and 5(i), is directed to patternmasking layers on at least a region of both the top and bottom of thesubstrate 103. If the capping layer selected is an ultrathin substrate,both sides of the substrate mold will be coated with tube material,which can be patterned using photolithography and etching, for example.If an SOI capping layer is used, the silicon will be thinned back to theburied oxide layer, in which case the buried oxide may be patterned. Ifa thick plain substrate is used, the substrate will be thinned back andan appropriate masking layer will have to be deposited and patterned onthe masking surface. These masking layers protect the substrate instrategic areas from removal during the device release etch.

At this step one may also deposit and pattern other materials on thesurface of the substrate mold. For example, the inventors have depositedthin films of platinum with a thin titanium adhesion layer to serve asheaters and resistive temperature sensors through methods known to thoseof ordinary skill in the art. Specifically, physical vapor depositionusing electron-beam evaporation was employed.

Once the substrate 103 is exposed in strategic areas, the substrate maybe exposed to a substrate etchant for a timed etch. FIGS. 5( j) and 5(k)show the final released tube structure. FIG. 5( k) is a cross section ofthe device of FIG. 5( j) at positions I-I, II-II, III-III and IV-IV. Theetch is controlled, for example through timing, such that the thinsubstrate walls are removed completely while leaving substrate in areaswith thick or masked substrate walls. For silicon substrates a number ofsilicon etchants have been used with good results, including potassiumhydroxide solution, xenon difluoride, fluorine, nitric acid/hydrofluoricacid mixtures, etc. Two fluid conducting tubes 105 and 106 are shown.For ease of presentation, going forward, the discussion will focus onfluid conducting tube 105. However, the following discussion appliesequally to tube 106. The fluid conducting tube has a thermallyinsulating inlet portion 105 b, a thermally insulating outlet portion105 a, and an intermediate portion 105 c. At least part of theintermediate portion 105 c contains at least one passive fluid stopvalve 111 and is encased in a thermally conductive region 102. The inletportion 105 b and the outlet portion 105 a are contacted by thermallyconductive structures 107. The fluid conducting tube 105 is encasedwithin the substrate 103 and the inlet portion 105 b and the outletportion 105 a may be accessed by way of ports 104.

Catalyst may be deposited inside the tubes using a variety of methods,including, for example, wet impregnation techniques, CVD or plating.

Importantly, through the DRIE etching process described above, any partof the substrate mold can be made to remain integrated with the tubematerial. Through this process the thermally conductive structures areintegrated with the thermally insulating tubes. Further, the substratematerial can be selected to remain on one, two, three, or all four sidesof the tube material.

The thickness of the substrate material integrated with the tubes can becontrolled through controlling thickness of top/bottom capping layersand the thickness and shape of the pattern in FIGS. 5( d) and 5(e). Thedepth of the tubes may also be controlled by controlling the thicknessof the substrate to be patterned in FIGS. 5( d) and 5(e) and/oradjusting the etch depth.

The ability to leave, in a controlled way, portions of the substratemold integrated with the tubes themselves is a substantial improvementover the prior art. This is achieved by including well defined releasepits in the original mold substrate in areas of the mold that are to beremoved and by masking substrate surfaces with a desired pattern inareas of the mold that are to be retained in the final micromachineddevice. It is to be understood that while FIGS. 5( a)-(k) address amicromachined device, the process of using a structured mold to formintegrated structures, which are structures that remain together aftermaterial removal steps, may be useful for a variety of non-micromachinedapplications.

FIG. 6( a) depicts a top view of an embodiment of the present inventionshowing the utilization of vacuum packaging 12, reflective coating 31,and low emissivity coating 32, the benefits of which have been set forthabove. FIG. 6( b) is a side view of the embodiment of FIG. 6( a) furthershowing the use of a packaging layers 33 above and below the devicelayer 1 a.

FIG. 7 depicts an embodiment of a thin-walled tube of the presentinvention. In this embodiment, the fluid conducting tube 5 issubstantially U-shaped and possesses an inlet portion 5 b, an outletportion 5 a and an intermediate portion 5 c.

Example 1

FIG. 8( a) and FIG. 8( b) show the results of a finite elementsimulation of fluid and heat flow in one fluid conducting tube 205 in amicromachined device constructed according to the present invention andadapted for combusting a fuel. This particular simulation models theheat exchange for a device burning 1 watt of a stoichiometric butane-airmixture at 1652° F. (900° C.) with the substrate held at 86° F. (30°C.), neglecting losses from the exterior surface of the tube andthermally conductive structures. Note, however, that any combustiblefuel may be used. The cold stream of fluid enters the inlet portion 205b of the fluid conducting tube 205 from the substrate 203, is preheatedby the fluid exiting the thermally conductive region 202, combusts inthe intermediate portion of the fluid conducting tube (which is encasedin the thermally conductive region 202), and returns to the substrate203 via the outlet portion 205 a of the fluid conducting tube 205. Thesesimulations indicate that between 50-70% of the heat in the hot streamexiting the thermally conductive region can be recovered using thisdesign and the recovered heat may be used to preheat the entering streamover the 3 mm long heat recovery section of the fluid conducting tube205. Calculations also indicate that the conductive loss through thetube is approximately 0.1 W at a thermally conductive region operatingtemperature of 1652° F. (900° C.).

Example 2

Theoretical analysis of the behavior of the thermally conductive regionas an ammonia cracker has been carried out. Results of this analysis areprovided in FIG. 9. The system was assumed to use the combustion ofhydrogen from a fuel cell anode to supply the energy to achieve elevatedtemperatures in the thermally conductive region for the endothermicammonia cracking reaction. It was assumed that enough catalyst would bepresent to crack 30 sccm of ammonia at 1652° F. (900° C.). Thetemperature of the thermally conductive region was varied to reflect theneed for higher cracking temperatures at higher ammonia flow rates. Anoptimal operating point for the device based on the increased importanceof conductive thermal losses for lower power operation and poorrecuperation of exhaust heat at higher power levels and ammonia flowrates is expected.

Example 3

As depicted in FIG. 10( a), one particular approach to using thedescribed gas-phase reactor of the present invention in a portable powerapplication is the thermal decomposition of a fluid reactant into fluidproducts. In this example the fluid reactant is ammonia gas and thefluid products are hydrogen and nitrogen for use in a fuel cell.Hydrogen is the preferred fuel of many fuel cell systems. It is,however, difficult and dangerous to store and transport. This approachallows ammonia to be transported, which results in increased energystorage density.

The ammonia decomposition approach would use the thermally insulatinginlet portion 5 b of a fluid conducting tube 5 to carry ammonia gas intointermediate portion 5 c of the fluid conducting tube 5. Theintermediate portion 5 c is encased in the thermally conductive region2, which is comprised of silicon or another material of high thermalconductivity. The thermally conductive region 2 is maintained at anelevated temperature, causing the ammonia gas to decompose (possiblywith the assistance of a catalyst in the intermediate portion 5 c). Thematerial of which the thermally conductive region 2 is composed isselected so that the region 2 is substantially isothermal duringoperation of the device of FIG. 10( a). The thermally insulating outletportion 5 a of the fluid conducting tube 5 carries the nitrogen andhydrogen products of the decomposition of the ammonia back to thesubstrate 3, where they would be directed to a fuel cell (not shown).The hydrogen not consumed by the fuel cell may be combined with air andfed back through the thermally insulating inlet portion 6 b of a secondfluid conducting tube 6 into the intermediate portion 6 c of the secondfluid conducting tube 6. As shown in FIG. 10( a), the second fluidconducting tube is encased in the thermally conductive region 2 so thatthe intermediate portions 5 c and 6 c are adjacent. The hydrogen fedthrough the second tube 6 would react with air (possibly with theassistance of a catalyst) in the intermediate portion 6 c to producewater and heat. The heat generated by combustion of the hydrogen wouldbe used to maintain the elevated temperature of the thermally conductiveregion 2 to supply the energy absorbed by the decomposition of theammonia to generate additional hydrogen for the fuel cell.

Example 4

FIG. 10( b) depicts an embodiment of the present invention constructedas in FIG. 10( a) and used for portable power generation, but carryingout the steam reforming of methanol. As with ammonia cracking in Example3 above, hydrogen is the desired product. The system design may be verysimilar to that for ammonia decomposition, but a 1:1 methanol-watermixture is the fluid reactant from which hydrogen and carbon dioxide areprovided as fluid products. Again, some portion of hydrogen not consumedby the fuel cell would be combined with air and used to maintain theelevated temperature of the thermally conductive region 2 and supplyheat to the reaction.

Example 5

FIG. 10( c) depicts the partial oxidation of butane as carried out by anembodiment of the present invention utilizing one fluid conducting tube.Butane is partially oxidized resulting in the generation of hydrogen andcarbon monoxide. The products may then be transported to and used in afuel cell.

Example 6

As shown in FIG. 10( d), another approach to produce electricity usingthe described device in a portable power application is to carry out theexothermic combustion of air and butane in the device. It is noted thatthe embodiment depicted in FIG. 10( d) uses thermoelectric elements 28.A mixture of butane and air undergoes combustion, forming water andcarbon dioxide.

Example 7

A key feature of the micromachined device of the present invention isits ability to thermally insulate a high-temperature reaction from itsenvironment. The reactor/heat exchanger of the present invention, asillustrated by the thermally conductive region and thermallycommunicating tube design of FIGS. 10( a)-(d), when properly packagedunder vacuum, will only dissipate a small fraction of its heat to theenvironment. To test the importance of vacuum packaging and to identifythe level of vacuum needed, a series of tests were performed in whichknown voltages were applied across a resistive heater to heat thethermally conductive region at various ambient pressures. The deviceused is shown in FIG. 13( a). Because current was measured along theheater, heater resistance could be calculated, and the temperature couldbe deduced as the resistance is a function of the temperature of thereactor. These tests provide insight on the heat transfercharacteristics of the reactor/heat exchanger, as illustrated in FIGS.10( a)-(d).

FIG. 11 summarizes the results of the tests. Below approximately 40mTorr, ambient pressure has little effect on the temperature of thereactor and, consequently, on the steady-state heat loss of the system.Above approximately 40 mTorr, however, increased ambient pressureresults in increased heat loss to the air. FIG. 11 thus verifies theimportant role that vacuum packaging plays in the present invention.

Example 8

FIG. 12( a) and FIG. 12( b) show fluid conducting tubes 305 and 306 withiridium catalyst deposited on the inside walls 305 g and 306 g. Catalystdeposition techniques set forth above were employed. Thermallyconductive structures 307 are also shown in these figures. Thesepictures confirm that catalyst is effectively being deposited internalto the tubes.

FIG. 12( c) depicts the results of a test conducted with an ammonia flowrate of 4 sccm. In the graph, the signal from a quadropole massspectrometer (QMS) is plotted over time for hydrogen, nitrogen, andammonia. The voltage across the integrated heater was increasedperiodically to increase the reaction temperature. The graph shows thatthe conversion of ammonia (represented by the NH₃ curve) to hydrogen andnitrogen (represented by the H₂ and N₂ curves respectively) increases asthe voltage across the heater is increased (corresponding to an increasein temperature). At the highest voltage setting, conversion of theammonia is approximately 35%. This translates into approximately 2 sccmof H₂ produced. The estimated temperature at the highest voltage isapproximately 1832° F. (1000° C.).

Example 9

FIGS. 13( a) and 13(b) provide images of a micromachined device 1. FIG.13( a) provides an image of the fully fabricated device 1 at roomtemperature. The suspended thermally conductive region 2 is freestanding and the tubes 5 and 6 are U-shaped to minimize stresses duringthermal expansion. FIG. 13( b) shows the device with the thermallyconductive region 2 heated to about 1832° F. (1000° C.) using theresistive heater trace 9, which is also used as a temperature sensor forthe temperature of the thermally conductive region 2. The amount ofpower required to bring the thermally conductive region 2 to 1832° F.(1000° C.) in an air ambient is 1.4 W. This test was performed todetermine the behavior of the device 1 when subjected to a substantialthermal gradient. Significantly, only the thermally conductive region 2is glowing, with the adjacent fluid conducting tubes 5 and 6 beingsubstantially cooler. The surrounding substrate 3 remains essentially atroom temperature, as confirmed by measurements of a referencetemperature sensor 26 placed on the substrate 3. These resultsdemonstrate that the thermally conductive region 2 achieves the highthermal isolation required for fuel processing and the substrate thermalgradient that is required for co-location of sensing, computing,actuation, and power generation in a single micromachined device.

It is to be understood that the present description illustrates certainaspects of the invention relevant to a clear understanding of theinvention. Certain aspects of the invention that would be apparent tothose of ordinary skill in the art and that, therefore, would notfacilitate a better understanding of the invention have not beenpresented in order to simplify the present description. Also, althoughthe present invention has been described in connection with certainembodiments, those of ordinary skill in the art will, upon consideringthe foregoing description, recognize that many modifications andvariations of the invention may be employed. It is intended that allsuch variations and modifications of the invention are covered by theforegoing description and the following claims.

1. A micromachined device for thermal processing at least one fluidstream, the micromachined device comprising at least one fluidconducting tube, wherein at least a region of the fluid conducting tubehas a wall thickness of less than 50 μm.
 2. The micromachined device ofclaim 1, wherein the fluid conducting tube provides an inlet portion, anoutlet portion and an intermediate portion intermediate the inletportion and the outlet portion.
 3. The micromachined device of claim 1,wherein the fluid conducting tube comprises silicon nitride.
 4. Themicromachined device of claim 1, wherein at least a region of the fluidconducting tube has a wall thickness less than 5 μm.
 5. Themicromachined device of claim 1, wherein at least a region of the fluidconducting tube has a wall thickness of 0.1-3 μm.
 6. The micromachineddevice of claim 1, wherein the micromachined device includes at leastone inlet portion for introducing a fluid into the fluid conductingtube.
 7. The micromachined device of claim 1, wherein the micromachineddevice includes at least one outlet portion for conducting a fluid fromthe fluid conducting tube.
 8. The micromachined device of claim 1,wherein the fluid conducting tube has a stress-relieving shape.
 9. Themicromachined device of claim 1, wherein the fluid conducting tube isgenerally U-shaped.
 10. The micromachined device of claim 1, wherein atleast one post is disposed within the fluid conducting tube.
 11. Themicromachined device of claim 10, wherein the post comprises a catalyst.12. The micromachined device of claim 1, wherein at least one staticfluidic mixing structure is disposed within the fluid conducting tube.13. The micromachined device of claim 1, wherein at least one passivefluidic stop valve is disposed within the fluid conducting tube.
 14. Themicromachined device of claim 2, further comprising a substrate andwherein the fluid conducting tube includes one or more regions of theinlet and outlet portions disposed in the substrate.
 15. Themicromachined device of claim 1, wherein a catalyst is disposed withinthe fluid conducting tube.
 16. The micromachined device of claim 2,wherein a catalyst is disposed within the intermediate portion of thefluid conducting tube.
 17. The micromachined device of claim 1, furthercomprising a sensor.
 18. The micromachined device of claim 1, furthercomprising an actuator.
 19. The micromachined device of claim 1, furthercomprising a substrate defining a sealed cavity, wherein substantialportions of the fluid conducting tube are mounted within the sealedcavity.
 20. The micromachined device of claim 1, further comprising athermoelectric device.
 21. The micromachined device of claim 1, whereinthe micromachined device is a component of a thermophotovoltaic device.22. The micromachined device of claim 1, wherein the micromachineddevice is a component of a portable power generator.
 23. Themicromachined device of claim 1, wherein the micromachined device is arefrigeration device.
 24. A micromachined device for processing at leastone fluid stream, the micromachined device comprising: at least onefluid conducting tube; and at least one thermally conductive structurein thermal communication with a first thermally insulating portion ofthe fluid conducting tube and a second thermally insulating portion ofthe fluid conducting tube.
 25. The micromachined device of claim 24,wherein the first thermally insulating portion of the fluid conductingtube is an inlet portion and the second thermally insulating portion ofthe fluid conducting tube is an outlet portion.
 26. The micromachineddevice of claim 24, wherein the thermally conductive structure comprisessilicon.
 27. The micromachined device of claim 24, wherein at least aregion of the fluid conducting tube has a wall thickness of less than 50μm.
 28. The micromachined device of claim 24, wherein at least a regionof the fluid conducting tube has a wall thickness of less than 5 μm. 29.The micromachined device of claim 24, wherein at least a region of thefluid conducting tube has a wall thickness of 0.1-3 μm.
 30. Themicromachined device of claim 24, wherein at least one post is disposedwithin the fluid conducting tube.
 31. The micromachined device of claim24, wherein the fluid conducting tube has a stress-relieving shape. 32.The micromachined device of claim 24, wherein a catalyst is disposedwithin the fluid conducting tube.
 33. The micromachined device of claim25, further comprising a substrate defining a sealed cavity, at least aregion of the inlet portion and at least a region of the outlet portiondisposed in the substrate, and wherein substantial portions of the fluidconducting tube are mounted within the sealed cavity.
 34. Amicromachined device for processing at least two fluid streams, themicromachined device comprising: a first fluid conducting tube; a secondfluid conducting tube; and at least one thermally conductive structurein thermal communication with a thermally insulating portion of thefirst fluid conducting tube and a thermally insulating portion of thesecond fluid conducting tube.
 35. The micromachined device of claim 34,wherein at least a region of at least one of the first fluid conductingtube and the second fluid conducting tube has a wall thickness of lessthan 50 μm.
 36. The micromachined device of claim 34, wherein at least aregion of at least one of the first fluid conducting tube and the secondfluid conducting tube has a wall thickness of less than 5 μm.
 37. Themicromachined device of claim 34, wherein at least a region of at leastone of the first fluid conducting tube and the second fluid conductingtube has a wall thickness of 0.1-3 μm.
 38. A micromachined device forprocessing at least one fluid stream, the micromachined devicecomprising: a thermally conductive region; and at least one fluidconducting tube with at least one thermally insulating portion, at leasta portion of the fluid conducting tube disposed within the thermallyconductive region.
 39. The micromachined device of claim 38, wherein atleast one post is disposed within the portion of the fluid conductingtube disposed within the thermally conductive region.
 40. Themicromachined device of claim 39, wherein the posts are thermallyconductive.
 41. The micromachined device of claim 38, wherein thethermally conductive region is substantially isothermal during operationof the micromachined device.
 42. The micromachined device of claim 38,wherein the thermally conductive region comprises silicon.
 43. Themicromachined device of claim 38, wherein the portion of the fluidconducting tube disposed within the thermally conductive region isencased within the thermally conductive region.
 44. The micromachineddevice of claim 38, wherein at least a region of at least one of thefirst fluid conducting tube and the second fluid conducting tube has awall thickness of less than 50 μm.
 45. The micromachined device of claim38, wherein at least a region of at least one of the first fluidconducting tube and the second fluid conducting tube has a wallthickness of less than 5 μm.
 46. The micromachined device of claim 38,wherein at least a region of at least one of the first fluid conductingtube and the second fluid conducting tube has a wall thickness of 0.1-3μm.
 47. The micromachined device of claim 38, wherein at least one postis disposed within at least one fluid conducting tube.
 48. A method forprocessing at least one fluid stream, the method comprising: providing amicromachined device including at least one fluid conducting tube havinga thermally insulating inlet portion, and a thermally insulating outletportion, and at least one thermally conductive structure in thermalcommunication with the inlet portion and the outlet portion; introducinga stream of at least one fluid into the inlet portion of the fluidconducting tube; processing the fluid stream within the fluid conductingtube; and conducting thermal energy between the inlet portion and theoutlet portion through the thermally conductive structure.
 49. Themethod of claim 48, wherein the micromachined device further comprises athermally conductive region, at least a portion of the fluid conductingtube disposed within the thermally conductive region.
 50. The method ofclaim 48, wherein at least one fluid reacts within the fluid conductingtube to produce at least two fluid reaction products, the fluidcomprising ammonia and the fluid reaction products comprising hydrogenand nitrogen.
 51. The method of claim 48, wherein at least two fluidsreact within the fluid conducting tube to produce at least two fluidreaction products, the fluids comprising methanol and water and thefluid reaction products comprising carbon dioxide and hydrogen.
 52. Themethod of claim 48, wherein at least two fluids react within the fluidconducting tube to produce at least two fluid reaction products, thefluids comprising air and butane and the fluid reaction productscomprising water and carbon dioxide.
 53. The method of claim 48, whereinat least two fluids react within the fluid conducting tube to produce atleast two fluid reaction products, the fluids comprising air and butaneand the fluid reaction products comprising hydrogen and carbon monoxide.54. The method of claim 49, further comprising: providing a second fluidconducting tube having an inlet portion, and an outlet portion, at leasta portion of the fluid conducting tube disposed within the thermallyconductive region; directing at least a portion of the fluid reactionproducts from the outlet portion of the fluid conducting tube to a fuelcell; directing at least a portion of fluids exiting the fuel cell tothe inlet portion of the second fluid conducting tube; and reacting theportion of the fluids exiting the fuel cell within the second fluidconducting tube to produce thermal energy and heat the thermallyconductive region.
 55. The method of claim 48, further comprisingproviding a catalyst within at least a region of the fluid conductingtubes.
 56. A portable power generator comprising: a micromachined deviceincluding at least one fluid conducting tube, and at least one thermallyconductive structure in thermal communication with a first thermallyinsulating portion of the fluid conducting tube and a second thermallyinsulating portion of the fluid conducting tube; and a fuel cell influid communication with the fluid conducting tube.
 57. A method forgenerating power comprising: providing a micromachined device includingat least one fluid conducting tube, and at least one thermallyconductive structure in thermal communication with a first thermallyinsulating portion of the fluid conducting tube and a second thermallyinsulating portion of the fluid conducting tube; providing a fuel cellin fluid communication with the fluid conducting tube; producing a fuelwithin the fluid conducting tube; and conveying the fuel to the fuelcell.
 58. The method of claim 57, wherein the fuel comprises hydrogen.59. The method of claim 57, wherein producing a fuel comprises reactinga stream of at least one fluid within the fluid conducting tube toproduce at least two fluid reaction products, the fluid comprisingammonia and the fluid reaction products comprising hydrogen andnitrogen.
 60. The method of claim 57, wherein producing a fuel comprisesreacting a stream of at least two fluids within the fluid conductingtube to produce at least two fluid reaction products, the fluidcomprising methanol and water and the fluid reaction products comprisingcarbon dioxide and hydrogen.
 61. The method of claim 57, whereinproducing a fuel comprises reacting a stream of at least two fluidswithin the fluid conducting tube to produce at least two fluid reactionproducts, the fluid comprising air and butane and the fluid reactionproducts comprising hydrogen and carbon monoxide.
 62. A method forfabricating a device for processing at least one fluid stream, whereinthe method comprises: patterning a substrate to form at least one tubemold having walls accessible to an environment and to form at least onerelease pit not accessible to the environment; depositing a thin film tocoat the walls of the tube mold; and removing selected regions of thesubstrate using a chemical etchant to form at least one fluid conductingtube.
 63. The method of claim 62, further comprising defining a wallthickness of at least a region of the fluid conducting tube bycontrolling a thickness of the thin film deposited on the walls of thetube mold.
 64. The method of claim 62, further comprising providing thefluid conducting tube with at least one open end disposed in thesubstrate.
 65. The method of claim 62, wherein the etch results indiscontinuous portions of the substrate remaining in thermalcommunication with the fluid conducting tube.
 66. The method of claim62, wherein removing selected regions of the substrate provides at leastone thermally conductive structure from the substrate, the thermallyconductive structure in thermal communication with a thermallyinsulating first portion of the fluid conducting tube and a thermallyinsulating second portion of the fluid conducting tube.
 67. The methodof claim 62, wherein removing selected regions of the substrate providesat least one thermally conductive structure from the substrate, whereinthe thermally conductive structure is in thermal communication with athermally insulating portion of a first fluid conducting tube and athermally insulating portion of a second fluid conducting tube.
 68. Themethod of claim 62, wherein a portion of the fluid conducting tuberemains disposed in an unetched thermally conductive portion of thesubstrate.
 69. The method of claim 62, wherein the device includes postswithin the fluid conducting tube.
 70. The method of claim 62, whereinthe posts comprise substrate material.
 71. The method of claim 62,wherein patterning the substrate provides structures within the tubemold that are static fluidic mixing structures in the fluid conductingtube.
 72. The method of claim 62, wherein patterning the substrateprovides structures within the tube mold that are passive fluidic stopvalves in the fluid conducting tube.
 73. The micromachined device ofclaim 1, wherein the fluid conducting tube comprises at least onejunction where one tube is connected to at least two tubes.
 74. Themicromachined device of claim 30, wherein the posts are thermallyconductive.
 75. The method of claim 57, wherein the fuel compriseshydrogen and carbon monoxide.